The present invention relates to a flexible tubular pipe for transporting fluids that is used in the offshore oil production field. It relates more particularly to a flexible pippe comprising an internal carcass with high corrosion resistance, and to the manufacturing process thereof.
The flexible pipes targeted by the present invention are formed from an assembly of various concentric and superposed layers, and are said to be of the unbonded type since these layers have a certain freedom to move relative to one another. These flexible pipes comply with, among others, the recommendations of the normative documents API 17J “Specification for Unbonded Flexible Pipe” and API RP 17B “Recommended Practice for Flexible Pipe” published by the American Petroleum Institute. The constituent layers comprise, in particular, polymeric sheaths that generally provide a sealing function, and reinforcing layers intended to take up the mechanical forces and that are formed by windings of metal wires or strips or various tapes or sections made of composites.
Unbonded flexible pipes used most often in the offshore oil industry generally comprise, from the inside outward, an internal carcass consisting of a profiled stainless steel strip that is wound helically in a short pitch into turns that are interlocked with one another, said internal carcass mainly serving to prevent the pipe from collapsing under the effect of the external pressure, a polymeric internal sealing sheath, a pressure vault consisting of at least one interlocking metal wire wound helically in a short pitch, said pressure vault serving to take up the radial forces associated with the internal pressure, tensile armor layers formed by long-pitch helical windings of metal or composite wires, said armor layers being intended to take up the longitudinal forces undergone by the pipe, and finally an external sheath intended to protect the reinforcing layers from seawater. In the present application, the expression “short-pitch winding” is understood to mean any winding having a helix angle for which the absolute value is close to 90°, in practice between 70° and 90°. The expression “long-pitch winding” itself denotes any winding for which the helix angle is less than or equal, as an absolute value, to 55°.
A flexible pipe comprising an internal carcass is referred to as a rough-bore pipe since the innermost element is the internal carcass that forms a rough bore owing to gaps between the metal turns of the interlocked metal strip.
The internal carcass is in direct contact with the fluid flowing in the pipe. However, the hydrocarbons extracted from certain oil fields may be extremely corrosive. This is the case, in particular, for multiphase hydrocarbons comprising high partial pressures of hydrogen sulfide (H2S), typically at least 2 bar, and/or of carbon dioxide (CO2), typically at least 5 bar, and that also have a high concentration of chlorides, typically at least 50 000 ppm. Such fluids are generally highly acidic (pH<4.5). In addition, their temperature may exceed 90° C. Under these very harsh conditions, the internal carcass must be able to maintain its integrity over a service life of at least 20 years.
Furthermore, the flexible pipe must have a collapse resistance that is sufficient to enable it to withstand high external pressures, especially hydrostatic pressure when the pipe is submerged at great depth (1000 m or even 2000 m or more), or else the external contact pressures experienced during offshore laying and handling operations. Furthermore, it is desirable to limit the weight of the flexible pipe, and therefore in particular that of the internal carcass, especially for applications at great depth. This weight reduction makes it possible, among other things, to facilitate offshore laying, to achieve greater water depths and to reduce the manufacturing and laying costs. However, the collapse resistance of the internal carcass is an increasing function of the yield stress of the profiled strip of which it is constituted. This is why it is advantageous to seek to increase the yield stress of this strip, while making sure, however, that the corrosion resistance remains satisfactory.
Application WO 00/00650 and the normative document API RP 17B disclose flexible pipes for which the internal carcass is made of austenitic stainless steel, especially the grades AISI 304 (UNS S30400), AISI 304L (UNS S30403), AISI 316 (UNS S31600) and AISI 316L (UNS S31603). They also disclose solutions in which the internal carcass is made of duplex steel, especially the grade 2205 (UNS S31803), or else a nickel-based alloy, especially the alloy 825 (UNS N08825).
Moreover, application WO 2006/097112 discloses flexible pipes for which the internal carcass is made of nickel-depleted duplex steel (“lean duplex”) especially the grade 2101 (UNS S32101).
However, these solutions from the prior art are not satisfactory. Indeed, internal carcasses made of austenitic steel or of duplex steel are not sufficiently resistant to the very corrosive media described above. Those made of a nickel-based alloy are themselves afflicted by mechanical properties that are too low and are moreover very expensive.